Increasing the strength of structural alloys is highly desired as it allows thinner wall construction for load bearing structural members or for vessels used for containing pressurized fluids. Thinner wall construction can lead to significant economic incentives due to material, fabrication, transportation and erection cost savings. In other applications, high strength structural materials provide enabling technologies, for instance, structural steel components for ultra-deep water drilling and production of hydrocarbons. However, before the strength potential of a higher strength structural material or alloy can be fully utilized in engineering design, it is critical that the material possesses adequate toughness to resist brittle fracture. It is known to those skilled in the art that, in the case of structural alloys, reducing the alloy's grain size can enhance simultaneously both the strength and toughness properties.
There are a number of approaches adopted in the past to refine the grain size of structural alloys. All of these approaches are based on controlled nucleation and growth of fresh grains via thermal or thermo-mechanical means to alter the stability of phases and/or by making the existing phases unstable.
In one commonly used approach, for example, temperature or material-chemistry is changed to move the material from one phase region, across existing phase boundaries, into another phase region. Each of the phase regions may have one or more stable phases. In these processes, however, the phase boundary and the phase free energies are not fundamentally altered.
For instance, in one approach, refinement of the alloy grain size is achieved by inducing phase transformation via thermal cycling the alloy across phase boundaries. Such thermal cycling treatments have been used effectively for grain refinement in several Fe—Mn and Fe—Ni steels used in cryogenic applications. For instance, U.S. Pat. No. 4,257,808 describes a thermal cycling treatment method for producing ultra-fine grain structure in low Mn alloy steel for cryogenic service. The technical and scientific basis for thermal cycling treatment is also described in the publication, “Grain Refinement Through Thermal Cycling in an Fe—Ni—Ti Cryogenic Alloy”, S. Jin et al., Metallurgical Transactions A, vol. 6A, 1975, pp. 141-149. This thermal cycling method uses existing phase boundaries. The phase boundary is not altered, nor is the phase free energy changed.
U.S. Pat. No. 5,413,649 proposes cycling the temperature between different phase regions of one of the components in a composite material. This induces phase transformation in that component, and provides grain refinement and superplasticity. This method uses existing phase boundary. The phase boundary is not altered, nor is the phase free energy changed.
In another widely used approach in high strength low alloy steels, austenite grains are refined by multi-step controlled hot working process, such as hot rolling, at sufficiently high temperatures to induce dynamic and/or static recrystallization to progressively refine the initial coarse austenite grains. Since this involves simultaneous application of both heat and mechanical deformation, this approach is also known as thermo-mechanical treatment (TMT) or processing. In most instances of TMT processing, microalloying with grain growth restraining alloy additions such as Nb or mixtures of Nb, Ti are used to further control the recrystallization and subsequent growth of the recrystallized grain. Numerous patents and publications are in the art describing both the science and practice of this technology for designing commercially attractive alloys with superior structural properties. For example, technical publication, “Processing-Thermomechanical Controlled Processing” by I. Kozasu, pp. 183-217 in “Materials Science and Technology” series edited by R. W. Cahn et al. in volume 7 “Constitution and Properties of Steels” edited by F. B. Pickering and published in 1992 by VCH, New York, provides the mechanisms and processes related to TMT. U.S. Pat. No. 6,254,698 “Ultra-High Strength Ausaged Steels with Excellent Cryogenic Temperature Toughness” describes the use of specific TMT to produce ultra-fine austenite grains.
There are also other approaches for refining grain size. This includes the cold work followed by high temperature annealing to recrystallize the heavily deformed grains. There is no phase transformation involved in this case; new grains of the same crystal structure nucleate and grow to replace the heavily deformed, unstable grains from the cold work. Since this is a thermally activated process, higher temperatures accelerate the formation of new grains. For instance, U.S. Pat. No. 5,534,085 proposes forging an alloy at low temperature, then heating the alloy to high temperature where recrystallization occurs to release the stored strain energy, thus achieving a fine and uniform microstructure. This process does not involve phase transformation.
U.S. Pat. No. 5,080,727 proposes heating a plastically deformed material to high temperature that destabilizes the low temperature phase. This results in a fine microstructure due to phase transformation induced recrystallization (presumably with increased kinetics driven by the stored strain energy). This method uses existing phase boundaries. The phase boundary is not altered, nor is the phase free energies changed.
U.S. Pat. No. 6,042,661 proposes changing the material chemistry to move it from an initial phase region into a different phase region, thus inducing phase transformation that results in superplasticity. Again, this method uses existing phase boundaries. The phase boundary is not altered, nor is the phase free energies changed.
U.S. Pat. No. 3,723,194 proposes rapidly heating a material from its initial α state to a temperature inside the α+γ dual phase region, thus inducing instability that provides superplasticity. This method uses existing phase boundary. The phase boundary is not altered, nor is the phase free energies changed.
U.S. Pat. No. 5,087,301 proposes rapidly cooling a molten alloy to form a solid supersaturated with a specific solute. The alloy is subsequently heated to a higher temperature (presumably to provide solute atoms with sufficient diffusivity) at which the solute precipitates out in the form of intermetallic particles. This process does not involve phase transformation.
U.S. Pat. No. 4,466,842 proposes hot rolling steel when cooling from γ to α+γ dual phase regions. This results in fine grain size due to two simultaneous processes, which include the γ to α phase transformation and the strain induced γ recrystallization. This method uses an existing phase boundary. The phase boundary is not altered, nor is the phase free energy changed.
The limitation with current methods for grain refining is concerned with the conflicting requirements for efficient and uniform grain refinement: high nucleation rate for new grains and no grain growth. A high nucleation rate is promoted by high thermodynamic driving force. For this, a large temperature change, ΔT, is required. To avoid grain growth, the temperature change should be instantaneous. However, this is very difficult to achieve in practice in large components that typify commercial applications. For these components the temperature change is only gradual even with the state-of-the-art commercial heating or cooling processes. The gradual change in temperature results in nucleation of some new grains of the new phase at the early stages of this temperature change. Upon continued change in temperature, the alloy or material transitions more into the new phase primarily by the growth of the existing nuclei to fairly coarse sizes, which is favored over further nucleation. Thus, rapid heating or cooling of the material is required to fully take advantage of all the driving force resulting from temperature change to promote nucleation and discourage growth. However, due to the limitations of finite heating and cooling rates in actual practice, the smallest grain size achievable by state-of-the-art techniques is limited to about 10 micrometers for equiaxed grains. There is considerable technological interest in further refining the grains down to less than 10 micrometers, preferably to less than about 5 micrometers, and even more preferably to less than about 1 micron. A new material processing methodology without the aforementioned limitations of current techniques is required to produce grain size refinement to less than 10 micrometers.